They later took control of the security service building in the city, which lies some 90 miles from the Russian border.

Also on Saturday, gunmen seized a police station in the eastern city of Kramatorsk following a shoot-out.

Other groups began their occupation of security buildings in Donetsk and Luhansk last weekend.

Protesters in the region, which has a large ethnic Russian population, hope to follow in the footsteps of the Crimean Peninsula and force a referendum on joining the Russian Federation.

The area was a strong bastion of support for ousted President and Kremlin ally Viktor Yanukovych and many residents fear they will be suppressed under the new Western-friendly interim government in Kiev.

Moscow has also warned any Ukrainian military aggression against ethnic Russians will derail top-level talks on the crisis scheduled to take place in Geneva next week.

Western leaders have expressed concern that the latest unrest strongly echoes the events which led to Russia's annexation of Crimea last month following an overwhelmingly-backed referendum.

The US has also called on Moscow to withdraw its troops from Ukraine's eastern border, where Nato claims up 40,000 soldiers are now stationed.

The EU and US have imposed a number of sanctions on Russian and Ukrainian individuals, but are yet to directly target Russia's economy.